US 8191423 B2
Dual piezoresistive transducers formed into a single silicon die, are anodically bonded to a pedestal. Two separate pressure ports extend through a plastic housing. The port openings inside the housing are surrounded by a groove having a shape and size that accepts the pedestal. A thin, liquid adhesive is deposited into the groove and allowed to level out. The pedestal is placed into the adhesive and embeds itself therein. Adhesive overflow into the ports is avoided by dimensioning the groove and depositing an amount of adhesive that will fill the groove but not overflow when the pedestal is placed therein. Once the adhesive cures, the adhesive bond strength is greater due to the adhesive being in shear relative to the groove side walls and pedestal sidewalls. The grooved structure provides an apparatus and methodology for precise die mounting and media sealing.
1. A pressure sensor comprised of:
a plastic housing have a pocket with a bottom comprised of a substantially planar surface;
a pressure port extending through the bottom surface to an exterior surface of the housing;
a groove formed into the bottom surface and surrounding the pressure port where the pressure port extends through the bottom surface;
an adhesive within the groove;
a pedestal within the groove and at least partially embedded into the adhesive; and
a piezoresistive transducer (PRT) die, anodically bonded to the glass pedestal, the die having at least one piezoresistive transducer, responsive to pressure in the pressure port.
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glass frit bonding; and
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4. A pressure sensor comprised of:
a plastic housing have a pocket with a bottom comprised of a substantially planar surface;
a first pressure port extending through the bottom surface to an exterior surface of the housing;
a second pressure port extending through the bottom surface to the exterior surface of the housing;
a groove formed into the bottom surface whereby the first port and the second port are completely surrounded by the groove where the first and second ports extend through the bottom surface;
adhesive within and partially filling the groove such that adhesive completely surrounds each port;
a pedestal in the groove and at least partially embedded in the adhesive, the pedestal completely surrounding the first port and completely surrounding the second port, the pedestal being at least partially embedded within the adhesive; and
a piezoresistive transducer (PRT) die bonded to the pedestal, the die having a first PRT device responsive to pressure in the first port and a second PRT device responsive to pressure in the second port.
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As used herein, the term differential pressure sensor refers to a pressure sensor that measures or senses an amount by which the pressure existing in a liquid or gas differs from a reference pressure, typically atmospheric pressure. The device shown in
The pressure sensor 1 is comprised of a housing 2 with a pocket 3 formed by or defined by a vertical side wall 4 that wraps around the pocket 3. The pocket 3 has an open top and a planar or substantially planar bottom surface 5. Two small ports 6, which are visible in the cross-sectional view of
The adhesive 50 has an uncured liquid state and a substantially solid cured state. Uncured adhesive is applied to the top of the planar bottom surface 5 as a bead of liquid. The adhesive 50 is applied so that it completely surrounds both ports 6. The pedestals 9 with the PRT dies pre-attached to them are placed on top of the bead of adhesive 50 before the adhesive 50 cures. The pedestals 9 thus settle into the uncured adhesive 50. When cured, the adhesive 50 mechanically bonds the pedestals 9 to the planar bottom surface 5. It also provides a seal between the planar top surface 5 and the pedestal 9.
A viscous gel 75 partially fills the pocket, substantially as shown. The gel 75 protects the PRT dies but it also allows ambient pressure to be applied to the upward facing side of the PRTs, i.e., the side of the PRTs facing ambient pressure.
The gel 75 is viscous but flexible enough to allow ambient air pressure to exert a downward force on the top side of the PRT dies 8. The aforementioned ports 6 carry a media upwardly as shown, the pressure of which exerts an upward force on the PRT dies. The PRT dies are thus subjected to two forces: ambient pressure and the pressure inside the corresponding ports 6. The PRTs measure the difference between those two pressures and are thus referred to as differential pressure sensors.
A problem with the prior art pressure sensor shown in
The problem of adhesive overflow into the ports could be solved by increasing the space or distance between the ports 6 and the adhesive 50 but increasing the space between the ports 6 and the adhesive 50 would require the overall size of the device 1 to be increased. Unfortunately, the trend in all fields of electronics, including pressure sensing, is to make devices smaller rather than larger. A pressure sensor having two or more separate PRTs and which has a reduced size but which also can avoid or eliminate adhesive 50 blocking the pressure ports 6 would be an improvement over the prior art. Another manufacturing process advantage is that a single PRT die 60 with two PRTs can reduce eight bond pads to six bond pads 95, which also reduces the number of connecting wires to the die from eight to six.
The die 60 is bonded to a pedestal, not visible in
The pedestal, which sits within the groove, also surrounds both ports. Two pressure inlet ports 12 and 14 extend from one side of the housing 15 and carry pressurized fluids (liquids or gases) to corresponding pressure ports 35 and 40, not visible in
As shown in
The pocket 20 has a bottom surface 25, which is preferably planar or at least substantially planar. A first pressure port 35 extends through the housing 15 from a bottom and exterior surface 30 of the housing 15, upwardly (as shown in the figure) through the planar bottom surface 25. A second pressure port 40, which is laterally displaced from and preferably parallel to the first port 35 extends through the bottom and exterior surface 30 of the housing 15 upwardly through the planar bottom surface 25.
Two diaphragms 65 and 70 are formed into the one die 60, with each of the diaphragms 65 and 70 located above a corresponding pressure port 35 and 40. Each diaphragm 65 and 70 is therefore responsive to pressure in a corresponding pressure port 35 and 40 because of the seal provided around each diaphragm by the bond between the pedestal and the die 60. Each diaphragm has a corresponding piezoresistive transducer (See 91 and 92 in
The diaphragms 65 and 70 are sized, shaped and arranged such that they are functionally and structurally isolated from each other by relatively thick mounting boss areas 62 of the die 60 and which surround each diaphragm. The increased thickness of the mounting bosses 62 relative to the thin diaphragms 65 and 70 provide a mechanically robust mechanical interface and bonding surface to the glass pedestal 55. In a preferred embodiment, the pedestal 55 and die 60 are anodically bonded together. When viewed from the top as shown in
The groove 45 discussed above is shown in cross section in
Referring again to
The groove 45 dimensions, namely its width, depth, and cross-sectional shape are selected along with the volume of adhesive 50 deposited therein, such that when the pedestal 55 is embedded into the adhesive within the groove, no adhesive flows out of the groove 45 and onto the planar top surface 25. By forming a groove structure 45 in the planar bottom surface 25, and only partially filling it with a liquid adhesive, the pedestal 55 can be precisely located relative to the ports 35 and 40 while avoiding adhesive spill-over at the same time that an enhanced-strength bond is formed. Even more importantly, a single die 60 with two PRTs can be mounted into the groove 45 thereby reducing the overall size of the resultant apparatus 10.
When the housing 15 is assembled as shown in
A single pressure port 35 is formed in the injection-molded housing 15 and extends through the bottom 25 surface of the pocket 20 to an exterior surface 30. The bottom surface of the groove 45 surrounding the port 35 has post-like protrusions 80 formed into the groove 45, which extend upwardly from the bottom of the groove 45 toward the bottom surface 25 of the pocket 20. The post-like protrusions 80 effectively provide a platform on which a pedestal 55 can rest after adhesive is added to the groove 45. Stated another way, the protrusions 80 prevents the adhesive 50 from being squeezed out from beneath the pedestal and helps to insure that some adhesive will remain between the bottom of the pedestal and the bottom of the groove. In addition to providing a fixed-height resting platform for the pedestal 55, the protrusions 80 also provide additional vertical surfaces to which an adhesive 50 can adhere to. The protrusions 80 thus improve the shear strength of the bond between the adhesive 50, the pedestal 55 and the side walls of the groove 45.
In an alternate embodiment, the post-like protrusions 80 are replaced by a continuous rail 84, illustrated in
As can be seen in
In the preferred embodiment, the adhesive 50 has an initial liquid state during which the viscosity is between about 0.3 and about 100 kilopoise. In its cured state, the adhesive 50 is substantially solid.
The grooves 45 and 46 have dimensions selected to allow the pedestal 50 to have a loose fit when installed therein. Stated another way, the width of a groove should be greater than the width of the pedestal walls to allow the pedestal to be freely and easily inserted into the groove and to at least partially embed itself within adhesive inside the groove 45 or 46.
In a preferred embodiment the pedestal is comprised of glass. In an alternate embodiment the pedestal is comprised of silicon.
While the groove 45 depicted in the
In a preferred embodiment, the housing 15 including the groove 45 is formed from an injection molded plastic. In an alternate embodiment, the housing 15 is an injection molded plastic however, the groove 45 is machined. Material suitable for the housing includes thermal plastic polymer or “TPP”.
The silicon die 60, which can carry one, two or more piezoresistive transducers 91 and 92, is attached to the pedestal using any one of anodic bonding, glass-frit bonding, and fusion bonding.
The foregoing description is for purposes of illustrations only. The true scope of the invention is set forth by the appurtenant claims.